We propose to research and develop breath-by-breath, non-invasive cardiac output monitoring technology. As a result of a highly successful SBIR lab- on-a-chip gas analyzer program, low cost pneumotachometers can be compensated for density and viscosity changes so that metabolic functions, such as O2 consumption and CO2 production, can be routinely measured in real time. Thus, cardiac output techniques, previously considered too difficult or too impractical, may be feasible. We propose revisiting Kim's (8) single-breath technique to estimate arterio-venous CO2 content difference, using significantly more exhaled O2 and CO2 concentration points on the alveolar plateau to develop accurate relationships between Fao2, Faco2 and respiratory exchange ratio. The Fick equation yields pulmonary capillary blood flow, Qc. Since deadspace affects the relationship between exhaled and alveolar gasses, we propose a new estimation method based on Fletcher et al's (21) single-breath CO2-tidal-volume waveform analysis combined with a virtual patient iterative anatomical/physiological model that converges on measured Feo2 and Feco2. Pulmonary shunting will be accounted for by determining Spo2 with a pulse oximeter, and Satvo2 and Satco2 from blood pH models. Data from a preliminary proof-of-concept system has been encouraging, yielding cardiac outputs of 8 - 10 lpm for active adults, consistent with published data. Production devices are expected to cost <$1000 and weigh <1 lb. PROPOSED COMMERCIAL APPLICATIONS: The availability of cardiac output measurement to routinely monitor the large population currently without benefit of such monitoring could significantly reduce the huge aftercare costs and morbidity and mortality resulting from undiagnosed cardiac complications in noncardiac-related procedures. A lightweight, rugged device would be ideally suited for use in field environments such as the ambulance and MEDEVAC transport, as well as the doctor's office, clinic, the ER and ICU.